The invention relates to a proportioning device.
Proportioning devices are employed to proportion liquids at laboratories. They generally have a displacement device with a displacement member which, when shifted, causes a liquid or air column to be moved. They are specifically known in the following designs:
Proportioning devices operating according to the air-cushion principle have a piston-and-cylinder unit by means of which an air column can be shifted to draw liquid into a pipette tip or expel it therefrom. The piston-and-cylinder unit does not contact the liquid here. Only the pipette tip, which is mostly made of plastic material, is wetted and may be exchanged after use.
On the contrary, proportioning devices operating as direct displacers have a syringe which is filled with a sample liquid. The piston and cylinder of the syringe are wetted by the liquid so that the syringe mostly is replaced with a new syringe or is cleaned before another liquid is proportioned. The syringe is also made of plastic material in most cases.
No-piston proportioning devices, for example, have a pipette tip with a balloon-like end portion which is expanded to draw in liquid and is compressed to expel it. Known pipette tips are disposable.
Dispensers are proportioning devices which are able to repetitively dispense an amount of a liquid they received, in small sub-amounts.
Furthermore, there are multi-channel proportioning devices which comprise a plurality of proportioning devices to proportion several amounts of liquid at the same time.
Air-cushion, direct displacer, and no-piston proportioning devices can exhibit an invariable or variable volume to be proportioned. A variation of the volume to be proportioned is mostly achieved by varying the displacement of the displacement device. For this purpose, the path of shift of the piston may be varied, for example, or the degree of deformation of the balloon-like end portion may be varied or the displacement device may be exchanged.
The displacement device of manual proportioning devices is solely driven by the physical force of the user. This has the advantages below:
The operator receives a tactile return information. Each variation of the force required for actuation is noticed immediately. Thus, the operator can check whether the proportioning device operates correctly. The operator can vary the speed of liquid reception and delivery directly and with no delay. Also, the liquid may be dispensed in an open jet. At this point, a contact may be avoided between the proportioning device and a vessel for the liquid. Further, the proportioning device does not rely on an energy supply. It may be intuitively utilized by the user. Troublesome instructions or programming are unnecessary.
The disadvantage of manual proportioning devices is that working is tiresome because of the large force requirement. Working frequently with manual proportioning devices can lead to damage to a person's health.
Electric proportioning devices drive the displacement device by means of an electric driving motor. The operator has to actuate electric push-buttons or switches to control the operations. Such proportioning devices have the advantage that their operation does not require considerable force.
However, the disadvantage is that the user does not receive a direct return information about the forces acting in the system, e.g. when the load rises as the pipette tip or syringe is clogged. Also, dispensing the liquid in an open jet is only possible to a limited extent. Work has to be stopped when the accumulator or battery is empty. Changes to the speed of liquid reception and delivery require to be programmed. Changes are mostly impossible during the proportioning operation.
U.S. Pat. No. 5,389,341 discloses a motor-driven pipette with an actuating button in which shifting an actuating button controls the movement of a piston via an electronic control system. The shifting of the actuating button is queried via an electronic path-detecting sensor and the result of query is electronically converted into the driving movement of the displacement device via a stepped motor. This electric proportioning device has the previously described disadvantages.
Accordingly, it is the object of the invention to provide a proportioning device which makes it easier or possible for the operator to obtain a tactile return information about the forces that act, to vary the speed of reception and delivery of the liquid during proportioning, to dispense the liquid in an open jet, and to easily use it, and which reduces the force required for actuation as compared to manually operated pipettes.